The main function of any heat pump, including refrigerators, is to raise the temperature of a supply of heat. In an absorption cycle heat pump, this is caused to occur by lowering the temperature of another quantity of heat. The heat whose temperature is to be raised is applied to a boiler (or evaporator), thereby causing a working medium such as H.sub.2 O to evaporate. The vapor is then absorbed in an absorbent solution having a substantial boiling point elevation--this causes the heat to be released at higher temperature. The absorbent solution is then returned to its original concentration, ready for reuse, by the action of the heat whose temperature is to be lowered. That heat is applied to a generator, causing working medium to boil out of the solution at a substantial boiling point elevation, and finally the vapor condenses at its boiling point, releasing the heat which was input at the generator at a much lower temperature. The absorber and evaporator operate at approximately the same pressure, and the generator and condenser also operate at about the same pressure, but one which is substantially different from the absorber/evaporator pressure. When the generator/condenser pressure is higher than the absorber/evaporator pressure, the cycle is the conventional one found in refrigerators and air conditioners, and is herein called forward cycle: heat is input at the two temperature extremes, and is delivered at midpoint temperatures. Conversely, when the pressures are reversed, the resulting cycle is herein called reverse cycle: heat is input at mid-temperatures, and is rejected at both the highest cycle temperature and the lowest cycle temperature. Both cycles are known in the prior art--see for example U.S. Pat. No. 4,350,571 and application Ser. No. 06/188,527 filed Sept. 18, 1980 by D. C. Erickson.
The amount of temperature increase provided by an absorption heat pump, also called its temperature lift, is thus seen to be determined by the boiling point elevation of the absorbent solution. The net lift realized will be the boiling point elevation minus the heat exchanger temperature differentials; thus, practical machines require boiling point elevations on the order of 30.degree. C. or more. Although in principle almost any material will provide almost any degree of boiling point elevation, there is a practical limit imposed by the requirement that the absorbent solution transport the working medium from the absorber to the generator. Thus, the absorbent solution must have an acceptably large carrying capacity for the working medium at the high boiling point elevation condition, as otherwise excessively high solution circulation rates (and attendant high solution heat exchanger heat losses) would be experienced. The carrying capacity is proportional to the derivative of the solution concentration with respect to the boiling point elevation, and this is the quantity which must be acceptably large at high boiling point elevations.
In addition to high boiling point elevation and acceptable carrying capacity, the absorbent solution should be reasonably noncorrosive such that ordinary materials of construction can be used; it should not thermally degrade or decompose at high use temperatures; it must not freeze or crystallize at normally encountered use conditions; it should have acceptable liquid properties such as low viscosity for pumping, minimal foaming tendency, easily boil, etc: it should be relatively non toxic, non explosive, and nonflammable; it should be reasonably available; and it should have a low vapor pressure so as not to require rectification, as in NH.sub.3 -H.sub.2 O systems.
The composition described below satisfies all these criteria. For higher temperature absorption heat pumps, water is clearly the preferred choice for working medium. Although many absorbents have been proposed and used for H.sub.2 O in the past, they all introduce disadvantages when employed in high temperature absorption cycles. Most previous research has centered on the refrigeration or air conditioning applications of these cycles, not involving high temperatures. The lithium halides, H.sub.2 SO.sub.4, and NaOH all cause excessive corrosion to ordinary materials of construction above about 180.degree. C. Various organic absorbents such as the glycols are subject to thermal degradation, have undesirably high vapor pressures, and have undesirably low carrying capacity.
Prior art patents describing absorption cycle absorbent compositions include U.S. Pat. Nos. 2,802,344, 4,005,584, 4,018,694, 4,172,043, 4,251,382, and 4,272,389, and 2,986,525.